Mesonephric-Like Adenocarcinoma of the Endometrium: Diagnostic Advances to Spot This Wolf in Sheep's Clothing. A Review of the Literature

Mesonephric-like adenocarcinoma of the uterine corpus with focal sarcomatous differentiation: A case report

Mesonephric-like adenocarcinoma of the uterine corpus was first reported in 2016, and added as a new item to the 5th edition of the WHO classification in 2020. It accounts for approximately 1% of patients with uterine corpus cancer, being rare. The histology of this carcinoma varies, making pathological diagnosis difficult. A diagnosis is made at an advanced stage in comparison with other histological types of uterine corpus cancer, and the prognosis is reportedly poor with high-level malignancy. In addition, there are few case reports of mesonephric-like adenocarcinoma of the uterine corpus with sarcoma. In this study, we report a patient in whom preoperative chemotherapy was performed under a diagnosis of cervical carcinoma, but the resected specimen led to a diagnosis of sarcomatous component-mixed mesonephric-like adenocarcinoma of the uterine corpus after surgery.

Mesonephric and Mesonephric-like Adenocarcinomas of the Gynecologic Tract: A Case Series and a Review of the Literature

We sought to present and describe all cases of mesonephric adenocarcinoma (MNAC) and mesonephric-like adenocarcinomas (MLAs) at our institution. These cancers are rare, morphologically similar tumors of the female reproductive tract. In this case series, we present 13 new cases of MNAC/MLA that were identified at St. Luke's University Health Network from 2016 to 2024. Demographics, clinical characteristics, and pathologic findings were collected from chart review. There were 6 uterine, 5 ovarian, and 2 cervical MNAC/MLAs. At presentation, more than half of the patients presented at early stages with 7, 2, 3, and 1 diagnosed at stages I, II, III, and IV, respectively. All patients underwent upfront surgical resection and were recommended adjuvant therapy. One patient declined adjuvant treatment. At the time of writing, 9 of 13 patients have completed treatment and have no evidence of disease, 1 is alive with disease, 1 is currently undergoing treatment, and 2 died of disease. Median overall survival (OS) was 15 mo (95% CI: 2.2-27.8 mo). Current literature regarding MNACs/MLAs suggests an overall poor prognosis, with the majority presenting at advanced stages. This case series describes patients diagnosed with early-stage disease and reports on their histopathology, treatment regimens, and clinical outcomes. The majority of these patients are without recurrence after upfront treatment. Continued surveillance of these patients to determine long-term outcomes is necessary to further elucidate overall prognosis.

Mesonephric-like adenocarcinoma of the ovary: a case study

Mesonephric-like adenocarcinoma (MLA) is a rare and newly recognized subtype of ovarian and endometrial carcinomas, introduced in the 2020 World Health Organization Classification. This tumor likely originates from Müllerian-derived tissues and often mimics more common ovarian cancers, leading to frequent misdiagnosis. This case study details a 36-year-old woman who presented with urinary symptoms following a hysterectomy. Imaging revealed a significant left ovarian mass, initially misdiagnosed as carcinosarcoma. Pathological evaluation ultimately confirmed MLA, characterized by diverse architectural patterns and specific immunohistochemical markers. The patient underwent chemotherapy due to the locally advanced disease. This case highlights the diagnostic challenges of MLA and emphasizes the need for awareness among clinicians to prevent misdiagnosis. Given its aggressive nature and tendency to early recurrence, further research is essential for establishing standardized diagnostic criteria and treatment protocols for this rare malignancy.

Rarely Observed Histopathological Subtypes of Endometrial Malignant Tumors in a Tertiary Care Center: A Case Series

Introduction Endometrial cancer, one of the most prevalent gynecological malignancies, represents a significant contributor to global mortality and morbidity. The histological subtype of this cancer is pivotal in treatment planning and patient risk stratification. This case series, comprising seven cases, underscores the significance of rare endometrial cancer variants and the importance of ancillary studies, specifically immunohistochemistry, in comprehending and diagnosing the condition. Materials and methods The data of all the rare histological subtypes of malignant endometrial tumors encountered from April 2021 to April 2024 were collected. The data collected included age; risk factor; histological type; tumor, node, metastasis (TNM) stage; the International Federation of Gynecology and Obstetrics (FIGO) grade; P53; and mismatch repair (MMR) status. The histopathology specimens were reviewed using a Labomed microscope (LX500) manufactured in India. Images were taken and added wherever necessary, and the results were studied, analyzed, clinically correlated, and are shown in tables. Results Seven rare histopathological subtypes of malignant endometrial tumors were encountered in this period in this tertiary care center. This included mesonephric like adenocarcinoma, high-grade carcinosarcoma, carcinosarcoma with the serous and chondrosarcomatous components, clear cell carcinoma of endometrium, endometrial stromal sarcoma with smooth muscle differentiation, endometrial stromal sarcoma, and undifferentiated sarcoma with focal myxoid degeneration.  Conclusion Diagnosis of uncommon endometrial carcinomas proves to be challenging due to their various histological subtypes, most often with overlapping features. The cornerstone of effective care lies in collaboration with clinicians, radiologists, and pathologists to identify these lesions as early as possible using a multidisciplinary approach for optimal patient outcome management.

Estrogen/Progesterone Receptor Expression and Cancer Antigen 125 Level as Preoperative Predictors to Estimate Lymph Node Metastasis in Endometrioid Endometrial Cancer

Loss of estrogen receptor/progesterone receptor (ER/PR) in endometrial cancer (EC) is associated with tumor progression and poor outcomes. Elevated pretreatment cancer antigen 125 (CA 125) level is a risk factor for lymph node metastasis (LNM). We evaluated whether the combination of ER/PR expression and CA 125 level could be used as a biomarker to predict LNM. We retrospectively investigated patients with endometrioid EC who underwent complete staging surgery during January 2015 to December 2020. We analyzed ER/PR status using immunohistochemical staining, and quantified its expression using the sum of both ER/PR H -scores. Receiver operating characteristic curves were used to identify optimal cutoff values of H -score and CA 125 levels for predicting LNM. A nomogram for predicting LNM was constructed and validated by bootstrap resampling. In 396 patients, the optimal cutoff values of the ER/PR H -score and CA 125 were 407 (area under the receiver operating characteristic curve: 0.645, P =0.001) and 40 U/mL (area under the receiver operating characteristic curve: 0.762, P <0.001), respectively. Multivariate analysis showed that CA 125 ≥40 UmL (odds ratio: 10.02; 95% CI: 4.74-21.18) and ER/PR H -score <407 (odds ratio: 4.20; 95% CI: 1.55-11.32) were independent predictors. An LNM predictive nomogram was constructed using these 2 variables and our model yielded a negative predictive value and negative likelihood ratio of 98.3% and 0.14, respectively. ER/PR expression with pretreatment CA 125 levels can help estimate LNM risk and aid in decision-making regarding the need for lymphadenectomy in patients with endometrioid EC.

Mucinous cystadenoma and benign mesonephric-like proliferation in the ovary - Further evidence for clonal relationship

Mesonephric-like adenocarcinomas rarely occur in the uterus and the ovary. Benign mesonephric-like (ML) proliferations and hyperplasia have been described solely within the ovary. Pathogenetic data are very limited. We report a case with microscopic focus of benign ML-proliferation in association with mucinous cystadenoma in the ovary. The immunophenotype was distinct (mucinous tumor: focal weak nuclear positivity for PAX-8, CK 7, patchy cytoplasmic positivity for p16 and negativity for estrogen receptor, CD 10, TTF-1, p53 wildtype; mesonephric component: diffusely positive for PAX-8, CK 7, luminal CD 10, TTF-1, focal staining for estrogen receptor, patchy cytoplasmic for p16, p53 wildtype). On NGS-analysis there was clonal mutation of KRAS p.G12C. The data provide additional evidence for the concept of transdifferentiation (Müllerian tissue representing Wolffian/mesonephric features on histology and immunostaining) within the pathogenesis of mesonephric proliferation of the female genital tract and demonstrate the clonal relationship between these distinct morphologic components.

Comprehensive Immunohistochemical Analysis of Mesonephric Marker Expression in Low-grade Endometrial Endometrioid Carcinoma

Immunohistochemical markers shown to be useful in identifying/confirming mesonephric/mesonephric-like differentiation (MLD markers) include thyroid transcription factor (TTF1), GATA-binding protein 3 (GATA3), and cluster of differentiation 10 (CD10). Only a few studies have examined the expression levels of MLD markers in endometrial endometrioid carcinomas (EECs). This study aimed to analyze the frequency and pattern of MLD marker expression in low-grade EECs. We performed immunostaining for the detection of TTF1, GATA3, and CD10 expression in 50 low-grade EEC tissue samples and evaluated their staining proportion and intensity. Nine tumors (18.0%) expressed at least one MLD marker in varying proportions and intensities, and 2 of these tumors were positive for 2 MLD markers (TTF1/GATA3 and GATA3/CD10, respectively). Three (6.0%) tumors showed moderate-to-strong nuclear TTF1 immunoreactivity in ≤5% of the tumor cells. Five tumors (10.0%) had at least moderate nuclear GATA3 staining, and three of them displayed a staining proportion of ≥15%. Three tumors (6.0%) were focal (mean proportion, 15%) but strongly positive for CD10. Our findings indicate that a subset of EEC can express one or more MLD markers with varying staining proportions and intensities. Given that a diagnosis of uterine mesonephric-like adenocarcinoma should be established based on a combination of characteristic histologic features, unique immunophenotypes, and confirmed molecular findings, pathologists should not exclude EEC based only on the presence of focal immunoreactivity for MLD markers. Awareness of the atypical expression patterns of MLD markers in EEC helps pathologists avoid misdiagnosing EEC as a uterine mesonephric-like adenocarcinoma.

Mesonephric-like adenocarcinoma of the ovary

Mesonephric-like adenocarcinoma is a new class of rare subtypes of the female reproductive system. Its clinical symptoms are similar to other types of ovarian tumors. The diagnosis is based on pathological and immunohistochemical methods. The main treatment option is surgery combined with chemotherapy. Few cases have been reported at home and abroad. We reported a case of a 45-year-old woman with a cystic solid mass in the left adnexa. The postoperative pathological diagnosis was mesonephric-like adenocarcinoma of the left ovary and mature cystic teratoma (partial infiltration of the small intestine). This case had no specific clinical symptoms. Immunohistochemical findings showed positive results of GATA3, TTF1, CD10, ER, and PR. Paclitaxel and carboplatin chemotherapy were given after the operation. Currently, no specific criteria are available for diagnosis and treatment of the disease. This article aims to improve the understanding of clinicians in this disease and create a basis for clinical diagnosis and treatment.

Case Report: The first report of PPP2R1A mutations in mesonephric-like adenocarcinoma of endometrial carcinoma

We performed clinical treatment, histopathology, immunohistochemistry and molecular analyses. To compare with the published literature and have a reference overview. A 57-year-old woman and a 77-year-old woman presented with mesonephric-like adenocarcinoma of endometrium at an early clinical stage. The former had no deep myometrial infiltration and no regional lymph node involvement. The latter had deep myometrial infiltration, presence of LVSI and no regional lymph node involvement. Both of the tumor cells were positive for PAX8, GATA-3,CD-10,TTF-1,AE1/AEs,Ki67,P53 and P16 in immunohistochemical staining (IHC)Test. Primary tumors were examined for gene mutations by next generation sequencing. The former was identified KRAS mutation. The latter had KRAS,PIKCA and PPP2R1A mutations. To our knowledge, it is the first time that PPP2R1A(protein phosphatase 2,regulatory subunit A,α) mutation in MLA is reported in English literature.

Mesonephric-like adenocarcinoma of the female genital tract: possible role of KRAS-targeted treatment-detailed molecular analysis of a case series and review of the literature for targetable somatic KRAS-mutations

PURPOSE

Mesonephric-like adenocarcinomas (MLA) of the female genital tract represent a rare and relatively recently described neoplasm exhibiting characteristic morphologic and immunohistochemical findings commonly associated with a KRAS-mutation. Most cases display an aggressive clinical behavior, but knowledge about treatment approaches is limited, especially for targeting KRAS.

METHODS

We report a series of eight cases with a detailed molecular analysis for KRAS. These cases as well as the data of previously published cases with detailed information regarding KRAS-mutational events were reviewed for a potential targeted approach and its prognostic impact.

RESULTS

Both the uterine and ovarian MLA harbor a somatic KRAS-mutation in about 85% of the reported cases, affecting the hotspot codons 12 and 13. 15.7% of the endometrial and 15.6% of ovarian MLA are wild type for KRAS. A p.G12A-alteration was seen in 5.6% (5/89) of the endometrial and in 6.2% (2/32) of the ovarian tumors, for p.G12C in 7.9% and 6.2%, for p.G12D in 32.6% and 34.5% and for p.G12V in 36% and 37.5%, respectively. Very limited data are available regarding the prognostic impact of different mutational sites within the KRAS-gene without significant prognostic impact.

CONCLUSION

Because of a specific p.G12C-KRAS somatic mutation, only the minority of MLA (7.9% with uterine and 6.2% with ovarian primary) are potentially targetable by sotarasib in that rare but aggressive subtype of adenocarcinoma of the female genital tract. Until now, the different location of a somatic KRAS-mutation is of no prognostic impact.

The importance of molecular classification of endometrial carcinomas in clinical practice: how to apply it and difficulties in application

Classification of endometrial carcinomas (EC) based solely on histological features is not sufficient for the prognostic and therapeutic guidance of patients. Furthermore, the existence of EC in which the histological type cannot be determined clearly and the poor reproducibility of histological typing have led to difficulties in clinical management. However, molecular classification of EC is very promising because of the high reproducibility and good correlation with clinical outcome. Within the scope of “the Cancer Genome Atlas Project”, EC were divided into four different genomic subtypes, and molecular classification models for EC were developed based on these molecular subcategories. The prognostic differences between these molecular subgroups and the benefit for guidance for adjuvant therapy have been clearly demonstrated in studies. In this article, the importance of molecular classification for EC is discussed and its use in clinical practice is reviewed.

Molecular Classification of Endometrial Cancer and the 2023 FIGO Staging: Exploring the Challenges and Opportunities for Pathologists

This commentary explores the complexities of the FIGO 2023 staging system and the inclusion of The Cancer Genome Atlas's (TCGA) molecular classification in the management of endometrial cancer. It highlights the importance of histology as a prognostic tool, while scrutinizing the merits and demerits of its application to aggressive endometrial cancers. The commentary review sheds light on the recent introductions of lymphovascular space invasion (LVSI) and lymph node metastasis size in cancer staging. It outlines the difficulties in differentiating between synchronous and metastatic endometrial and ovarian cancers, underlining their implications on treatment strategies. Furthermore, the commentary discusses the integration of molecular classifications within the FIGO 2023 framework, emphasizing the pivotal yet challenging implementation of the pathogenic POLE mutation test. The commentary concludes by reaffirming the vital role of pathologists in executing the FIGO 2023 staging system.

Corded and hyalinized endometrioid carcinoma: Summary of clinical, histological, immunohistochemical and molecular data

Corded and hyalinized endometrioid carcinoma (CHEC) represents a potential pitfall for pathologists. This study aimed to provide a complete overview of all clinicopathological and molecular features of CHEC. Electronic databases were searched for all published series of CHEC. Clinical, histological, immunohistochemical and molecular data about CHEC were extracted and pooled. Six studies with 62 patients were identified; mean age was 49.8 years (range 19-83). Most cases showed FIGO stage I (68%), low grade (87.5%), and a favorable outcome (78.4%), with "no specific molecular profile" (NSMP). A subset of cases showed high-grade features (12.5%), p53 abnormalities (11.1%) or mismatch repair (MMR) deficiency (20%) and occurred at an older age (mean age>60 years). Common features of CHEC were: superficial localization of the corded component (88.6%), squamous/morular differentiation (82.5%), nuclear β-catenin accumulation (92%), partial/total loss of CKAE1/AE3 (88.9%), estrogen receptor (95.7%) and e-cadherin (100%), stromal changes such as myxoid (38.5%), osteoid (24%) and chondroid (4.5%), CTNNB1 mutations (57.9%), and POLE-wild-type (100%); 24.4% of cases showed lymphovascular space invasion. A minority of cases (16.2%) showed poor outcome despite a low-grade, NSMP phenotype; the molecular basis for the aggressiveness of these cases is still undefined. Further studies are necessary in this field.

Radiologic Findings of Mesonephric-Like Adenocarcinoma of the Uterine Corpus: A Case Report

According to the 2020 World Health Organization classification, mesonephric-like adenocarcinoma (MLA) is newly categorized as a subtype of endometrial carcinoma and remains a relatively unknown disease owing to its rarity. To the best of our knowledge, radiological findings of MLA have not been reported in the English literature. The uterine MLAs show a worse clinical prognosis and a more aggressive biological behavior than the usual endometrial carcinoma. Herein, we present the imaging findings of a 65-year-old female with a MLA in the uterine corpus. The tumor was a solid endometrial mass with deep myometrial invasion, poor contrast enhancement, and moderate diffusion restriction.

The Polish Society of Gynecological Oncology Guidelines for the Diagnosis and Treatment of Endometrial Carcinoma (2023)

BACKGROUND

Due to the increasing amount of published data suggesting that endometrial carcinoma is a heterogenic entity with possible different treatment sequences and post-treatment follow-up, the Polish Society of Gynecological Oncology (PSGO) has developed new guidelines.

AIM

to summarize the current evidence for diagnosis, treatment, and follow-up of endometrial carcinoma and to provide evidence-based recommendations for clinical practice.

METHODS

The guidelines have been developed according to standards set by the guideline evaluation tool AGREE II (Appraisal of Guidelines for Research and Evaluation). The strength of scientific evidence has been defined in agreement with The Agency for Health Technology Assessment and Tariff System (AOTMiT) guidelines for scientific evidence classification. The grades of recommendation have been based on the strength of evidence and the level of consensus of the PSGO development group.

CONCLUSION

Based on current evidence, both the implementation of the molecular classification of endometrial cancer patients at the beginning of the treatment sequence and the extension of the final postoperative pathological report of additional biomarkers are needed to optimize and improve treatment results as well as to pave the route for future clinical trials on targeted therapies.

Cytological features of mesonephric-like adenocarcinoma of the uterine corpus

BACKGROUND

Although several studies have documented the histological features of uterine mesonephric-like adenocarcinoma (MLA), its cytological features have been rarely reported.

METHODS

We searched for histologically confirmed uterine MLA cases in the pathology archives of three institutions between 2010 and 2021. All available cytology slides were examined to identify the cytological features of uterine MLA.

RESULTS

We included 16 patients with uterine MLA and reviewed the slides obtained from 21 cytology samples. Samples were obtained from the cervicovagina (9/21, 42.9%), peritoneal washing (8/21, 38.1%), pleural effusion (2/21, 9.5%), and transbronchial needle aspiration of mediastinal lymph node (2/21, 9.5%). Preparation methods included ThinPrep (11/21, 52.4%), SurePath (8/21, 38.1%), and conventional smear (2/21, 9.5%). Regardless of the sampling site and preparation method, cytology samples displayed tight three-dimensional cellular clusters showing monotonous, small-to-medium-sized, round, hyperchromatic nuclei, indistinct nucleoli, scant cytoplasm, and high nuclear-to-cytoplasmic ratio. Approximately half of the samples (10/21, 47.6%) showed hyaline-like globules. Mitotic figures (7/21, 33.3%) and apoptotic bodies (13/21, 61.9%) were also observed. No tumor diathesis or nuclear feathering was identified.

CONCLUSIONS

Irrespective of sampling site and preparation method, the majority of uterine MLA cases showed the following cytological features: tight three-dimensional cellular clusters showing small-to-medium-sized, round, hyperchromatic nuclei with indistinct nucleoli and high nuclear-to-cytoplasm ratio. In case a cytology sample suspicious of a glandular lesion displays these cytological features, which are distinct from those of endocervical adenocarcinoma, uterine MLA should be included in the differential diagnosis.

Mesonephric-Like Adenocarcinomas a Rare Tumor: The Importance of Diagnosis

Mesonephric-like adenocarcinomas (MLA) are rare neoplasms that arise in the uterine body and ovary and have been added to the World Health Organisation's recent 2020 classification of female genital cancers. The pathogenesis of MLA is unknown and it remains debated whether they represent mesonephric carcinomas (Wolffian) arising in the endometrium/ovary or endometrioid carcinomas (Müllerian) closely mimicking mesonephric carcinomas. Here we report the case of a 57-year-old woman with an initial misdiagnosis of endometrioid adenocarcinoma on diagnostic biopsy. The patient came to our clinical evaluation for the appearance of menometrorrhagia complicated by anemia for several months. Therefore, she underwent pelvic echo-flowmetry, with indication for diagnostic hysteroscopy with endometrial biopsy, which yielded a positive result for endometrioid endometrial adenocarcinoma. Following staging CT scan and targeted examinations on pulmonary findings, the patient underwent surgery with surprise of definitive diagnosis deponent for endometrial MLA. Our intention is to establish a brief review of the scientific evidence in the literature and the tools available for a correct histological diagnosis, in the light of the scant anatomopathological evidence. Our question gives rise to the motive for the publication: is immunohistochemistry the right way to resolve the diagnostic error at histology, which is usually the only source of diagnostic certainty? This case is intended to alert of diagnostic error that risked having the patient treated as a neoplasm with a favorable prognosis and low degree of aggressiveness instead of for a very aggressive and poor prognosis tumor such as MLA.

Mesonephric-Like Adenocarcinoma of Uterine Corpus: A Clinicopathological and Targeted Genomic Profiling Study in a Single Institution

Background

Mesonephric-like adenocarcinoma (MLA) is a recently characterized, rare, and aggressive neoplasm that mostly arises in the uterine corpus and ovary. MLA shows characteristic pathological features similar to mesonephric adenocarcinoma of the cervix. The origin of MLA is still controversial and recognition of it remains challenging for pathologists. The aim of this study was to enrich the clinicopathological features of MLA in the uterine corpus and explore its molecular alterations by targeted next-generation sequencing (NGS).

Methods

Four cases of MLA were identified among a total of 398 endometrial carcinomas diagnosed in our institution between January 2014 and December 2021. Immunohistochemistry and targeted NGS spanning 437 cancer-relevant genes were performed.

Results

The most common symptom was abnormal vaginal bleeding, and the average age was 68 years. Histologically, the tumors showed a mixture of varied growth patterns including papillary, glandular, tubular, cribriform, solid, and slit-like architectures, which were lined by columnar to cuboidal cells with overlapping vesicular nuclei and sometimes nuclear grooves. Intraluminal eosinophilic colloid-like secretions were focally evident in three of the four cases. Immunohistochemically, the MLAs were positive for GATA3 (4/4), TTF-1 (3/3), luminal CD10 (3/3), calretinin (2/3), and patchy P16 (3/3) and were negative for ER (0/4) and PR (0/4). The expression of P53 was “wild type” (4/4). By targeted NGS, 3/4 (75%), 2/4 (50%), and 1/4 (25%) cases harbored PIK3CA, KRAS, and PTEN mutations, respectively. None of the tumors had mutations in DNA mismatch repair genes, ARID1A/B, POLE, CTNNB1, SMARCA4, or TP53. At the time of diagnosis, three were presented with FIGO IB stage and one with IIIC stage. Two patients received postoperative chemotherapy and radiotherapy and they were alive without evidence of disease at 8 and 56 months follow-up, respectively. One patient developed pulmonary metastasis 13 months after surgery and chemotherapy, and one was dead of the disease 24 months after the operation without adjuvant therapy.

Conclusions

MLA is a rare and aggressive malignancy, representing approximately 1% of all endometrial carcinomas. It exhibits mixed architectures associated with distinctive immunophenotype and recurrent KRAS and PIK3CA mutations, supporting classified as of Müllerian origin with mesonephric differentiation.

Mesonephric-like Adenocarcinoma of the Female Genital Tract: From Morphologic Observations to a Well-characterized Carcinoma With Aggressive Clinical Behavior

Mesonephric-like adenocarcinoma (MLA) was introduced as a new tumor type in the endometrium and the ovary in the 2020 World Health Organization (WHO) Classification. This is a rare recently described (2016) and clinically aggressive carcinoma with a propensity for distant spread, especially to the lungs. MLA has a characteristic morphology and immunophenotype (hormone receptor negative; TTF1 and/or GATA3 positive). These neoplasms are commonly associated with KRAS and PIK3CA mutations and in the Cancer Genome Atlas (TCGA) molecular classification of endometrial carcinomas fall into the copy number low/no specific molecular profile category. Although they show significant morphological, immunophenotypic and molecular overlap with cervical mesonephric adenocarcinomas, there are other parameters which suggest a Mullerian origin and, as such, the term MLA seems apt. MLA can be added to the list of endometriosis-associated ovarian neoplasms. In this paper, I outline the series of events which lead to the first description of MLA and review the subsequent literature on this tumor type which has expanded on the morphologic features and immunophenotype, discovered the molecular underpinnings and elucidated the clinical behavior. The discovery of MLA represents an example of "new" entities still to this day being discovered through careful morphologic observations and referral of cases for specialist opinion.

Mesonephric-like Adenocarcinoma of the Uterine Corpus: Comprehensive Immunohistochemical Analyses Using Markers for Mesonephric, Endometrioid and Serous Tumors

Mesonephric-like adenocarcinoma (MLA) of the uterine corpus is a rare but distinct malignant tumor of the female genital tract, demonstrating a characteristic morphology and unique immunohistochemical profiles and molecular alterations. We conducted immunohistochemical staining (IHC) to make precise differential diagnoses of uterine MLAs from common histological subtypes of endometrial carcinomas. We collected 25 uterine MLAs and performed IHC for GATA3, TTF1, CD10, ER, PR, p16, p53, and HER2. Seventeen cases (68.0%) showed at least moderate nuclear GATA3 immunoreactivity in ≥25% of tumor cells. Most cases expressed TTF1 (17/21, 81.0%) and CD10 (luminal; 17/21, 81.0%). Heterogeneous TTF1 expression was noted in 12 cases. An inverse pattern of GATA3 and TTF1 staining was observed in eight cases (32.0%). Three cases (12.0%) showed moderate-to-strong ER expression in ≥25% of tumor cells, and two cases (8.0%) showed moderate-to-strong PR expression in ≥5% of tumor cells. These hormone receptor-positive MLAs varied in intensity and proportion of GATA3 staining. None of the 25 cases exhibited either diffuse and strong p16 expression or aberrant p53 expression. Five cases (20.0%) showed equivocal HER2 immunoreactivity (score 2+), but HER2 FISH confirmed that none of them exhibited HER2 gene amplification. In summary, a small subset of uterine MLAs displayed atypical IHC results: focal but strong expression of ER or PR, the complete absence of GATA3 immunoreactivity, the concurrent expression of mesonephric and hormone receptors, and the inverse pattern of GATA3 and TTF1 staining. These unusual immunophenotypes may complicate the differential diagnosis of MLA. Moreover, pathologists should be encouraged to interpret the IHC results cautiously.

Sustained response to lenvatinib and pembrolizumab in two patients with KRAS-mutated endometrial mesonephric-like adenocarcinoma

•Endometrial mesonephric-like adenocarcinoma (MLA) is a rare and aggressive subtype of epithelial endometrial cancer.•KRAS mutations are characteristic in patients with endometrial MLA.•We report two cases of KRAS-mutated endometrial MLA with excellent and durable responses to lenvatinib and pembrolizumab.•Tumors with KRAS mutations may be particularly sensitive to lenvatinib, but this warrants further study.

Mesonephric-like Differentiation of Endometrial Endometrioid Carcinoma: Clinicopathological and Molecular Characteristics Distinct from Those of Uterine Mesonephric-like Adenocarcinoma

When diagnosing endometrial carcinoma cases, we encountered histological features that strikingly resembled uterine mesonephric-like adenocarcinoma (MLA), but the differential diagnosis remained challenging after performing immunostaining. Considering the aggressive biological behavior and poor prognosis of uterine MLA, we believe that the accurate recognition of mesonephric-like differentiation (MLD) is important in the diagnosis of endometrial carcinoma. We aimed to investigate the clinicopathological and molecular characteristics of such cases and compared them with those of uterine MLAs. Five patients diagnosed with endometrioid carcinoma (EC) with MLD were included in this study. Histological evaluation, immunostaining, and targeted sequencing were performed. All five tumors showed typical morphological features of MLA, including densely aggregated tubular structures, deep basophilia under low-power magnification microscopy, eosinophilic intraluminal secretions, and diverse growth patterns. Immunostaining revealed moderate-to-strong nuclear immunoreactivity for estrogen and progesterone receptors in more than 50% tumor cells. The staining intensities and proportions of PAX2 and GATA3 were variable. None of the tumors harbored KRAS mutations. Considering the prognostic implications, ancillary tests, including immunostaining and targeted sequencing, should be performed to accurately differentiate between endometrial EC-MLD and uterine MLA.